Immobility and strength are still more ob viously required in that part of the spine by which the weight of a horizontal body is to be transferred to a single pair of extremities articulated to the trunk behind the centre of gravity. The anchylosis of the bodies of the vertebrae, which already begins to appear in the last dorsal, is, therefore, continued through all the sacral vertebrae as far as the caudal region ; and this consolidated mass (b to c) is united laterally to the iliac bones. Hence it is always difficult to determine the number of vertebra of which it is composed. We have made sections of the sacrums of many different birds with a view to determine this fact, and have generally found the number greater than that which is indicated in the tables of Cuvier. Thus the Stork has twelve, instead of eleven sacral vertebra ; the Coot thirteen, instead of seven ; the Kingfisher eleven, instead of eight : while the Ostrich, on the other hand, has but seventeen, instead of twenty bones of the sacrum. The bodies of the sacral vertebra are broad, but shallow, and towards the tail the floor of the vertebral canal is formed by a mere lamina of bone : the canal is remarkably dilated in this part of the spine for the enlargement of the cord which gives off the nerves to the posterior extremity. It is a curious fact that the roots of these nerves pass out of the osseous canal by separate orifices, the ganglion on the posterior root and the union of the two being external to the spine. The .aspect of all these orifices is la teral, in the intervals of the transverse pro cesses of the different vertebra, which are not united together as in the mammalia. The first four or five sacral vertebrae give off two sets of transverse processes, one ventral, the other dorsal ; the ventral ones are wanting in the succeeding four, and then suddenly reappear to abut against the symphysis of the ilium and ischium, and are so continued double to the end. The spinous processes which are prin cipally developed from the anterior sacral ver tebra, give off from their extremities lateral expansions, which anchylose with the iliac bones, and form an osseous roof, arching over and concealing the transverse processes.
The coccygeal vertebrae of birds, though never prolonged into a conspicuous caudal appen dage, are in general moveable upon each other, and are frequently nine in number. With the exception of the last, they are broad and short and perforated for the lodgement of the spinal marrow. With the exception of the last also they have spines on both the dorsal and ventral aspects ; and the anterior vertebrae have also transverse processes. The last caudal vertebra (d, fig. 125) is so singularly shaped, that were it found alone in a fossil state it would hardly be recognized as a bone of the spine. It has no medullary canal and no processes ; but is compressed laterally and terminates above and often also below in a sharp edge ; its posterior extremity is obtuse. It supports the coccygeal oil-gland, and affords a firm basis to the tail feathers, which, from their use in guiding the motions of the bird through the air, Linnaeus termed the rectrices.*
In the Toucan the three last caudal vertebra are anchylosed together ; the six anterior ones are articulated by ball and socket joints, the ball and the socket being most distinct in the two last of these joints ; that between the sixth and seventh vertebrae is provided with a capsule and synovial fluid, the others have a yielding ligamentous mode of connexion. The spinous processes of these vertebrae, both superior and inferior, are of moderate size, but smallest in the sixth, where the greatest degree of motion takes place ; the transverse pro cesses on the contrary are large and broad so as almost to preclude lateral motion. We have given a more particular description of these vertebra because of the singular move ments observable in the tail of the Toucan ; it can be inflected dorsad till the superior spines of the vertebrae are brought in contact with the sacrum ; and in the performance of this motion the lateral muscles, which at first tend rather to oppose the elevators, become, at a certain point of inflection dorsad of the centre of motion, elevators themselves, and thus com bining with the elevators jerk the tail upon the back ; it is thus that the tail turns as if on a hinge operated upon by a spring.
As the prehensile functions of the hand are transferred to the beak, so those of the arm are performed by the neck of the bird ; this portion of the spine is therefore composed of numerous, elongated, and freely moveable ver tebra, and is never so short or so rigid but that it can be made to apply the beak to the coccy geal oil-gland, and to every part of the body for the purpose of oiling and cleansing the plumage. In birds that seek their food in water it is in general remarkably elongated, whether they support themselves on the surface by means of short and strong natatory feet, as in the Swan, or wade into rivers and marshes on elevated stilts, as in the Crane, &c.
The articular surfaces of the bodies of the cervical vertebrae, like those of the dorsal series above mentioned, are concave in one direction and convex in the other, so as to lock into each other, and in such a manner that the superior vertebrae move more freely forwards, the middle ones backwards, while the inferior ones again bend forwards ; producing the ordinary sigmoid curve observable in the neck of the bird.
This mechanism is most readily seen in the long-necked waders which live on fish and seize their prey by darting the bill with sudden velocity into the water. In the common Heron, for example, (Arden cinerea) the head can be bent forward on the atlas or first vertebra, the first upon the second in the same direction, and so on to the sixth, between which and the fifth the forward inflection is the greatest; while in the opposite direction these vertebrae can only be brought into a straight line. From the sixth cervical vertebra to the thirteenth the neck can only be bent backwards ; while in the opposite direction it is also arrested at a straight line. From the fourteenth to the eighteenth the articular surfaces again allow of the forward inflection, but also limit the opposite motion to the straight line.